The subject matter of this application is related to the subject matter of British Patent Application No. GB 0100552.9, priority to which is claimed under 35 U.S.C. xc2xa7119 and which is incorporated herein by reference.
1. Field of the Invention
Aspects of the invention relate to determining rotor position in a switched reluctance machine.
2. Description of Related Art
The control and operation of switched reluctance machines generally are described in the paper xe2x80x9cThe Characteristics, Design and Applications of Switched Reluctance Motors and Drivesxe2x80x9d by J M Stephenson and R J Blake, delivered at the PCIM ""93 Conference and Exhibition held in Nurnberg, Germany, Jun. 21-24, 1993, which paper is incorporated herein by reference. In that paper the xe2x80x9cchoppingxe2x80x9d and xe2x80x9csingle-pulsexe2x80x9d modes of energization of switched reluctance machines are described for operation of the machine at low and high speeds, respectively. A typical prior art drive is shown schematically in FIG. 1 and one of the many known converter topologies is shown in FIG. 2, where a resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal.
More specifically, FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. To this end, a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms, and its output may also be used to generate a speed feedback signal.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. FIG. 2 shows a configuration for a single phase of a polyphase system in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the xe2x80x9cDC linkxe2x80x9d of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the xe2x80x9cDC link capacitorxe2x80x9d, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called xe2x80x9cripple currentxe2x80x9d), which cannot be drawn from or returned to the supply. In practice, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer mounted in relation to the machine rotor shaft, as referenced above and as shown schematically in FIG. 1, in which a rotating disk is mounted on the machine rotor shaft which co-operates with a fixed optical or magnetic sensor. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization. A significant property of such a device is that it functions at zero rotor speed, allowing the control circuit to identify the correct phase(s) to energize to provide torque in the desired direction.
This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability. Various methods for dispensing with the rotor position transducer have been proposed, and several of these are reviewed in xe2x80x9cSensorless Methods for Determining the Rotor Position of Switched Reluctance Motorsxe2x80x9d by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, Sep. 13-16 1993, Vol. 6, pp 7-13, incorporated herein by reference.
Many of these methods proposed for rotor position estimation use a measurement of phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. This characteristic can be stored as a flux-linkage/angle/current table and is depicted graphically in FIG. 3.
In general, these methods require the machine to be rotating for the position-determining algorithms to function correctly. Starting from rest generally requires a quite different technique to run the machine up to some appropriate speed so that the algorithms can take over. For example, EP-A-1014556 (Green), incorporated herein by reference, describes a method of injection of pre-determined pulses of flux linkage into two phases in order to compile sufficient data to interrogate a stored table of current and rotor angle. This method requires a control system with sufficient capacity to capture current readings simultaneously and with the capability to integrate the applied voltage.
Similarly, U.S. Pat. No. 6,107,772 (Dana), incorporated herein by reference, discloses a method of injecting current into three phases of a polyphase machine and performing a series of comparisons on the results of measurements of the times taken for the three currents to traverse between two predetermined levels. This method requires significant computing ability and the capability to store the results of intermediate steps.
In one embodiment, the present invention provides a method of determining rotor position at zero speed by timing the rise of current in phases to a predetermined level. The rise time of phase current is directly related to the inductance of the magnetic circuit for that phase and rotor position, since the inductance is determined by the position of the rotor poles relative to the stator poles for that phase. The invention in this embodiment injects current coincidentally into two phases. A simple look-up table, storing rotor angles for ordinates of time, can be used to allow the rotor position to be determined by comparing the possible rotor positions according to each reading for substantial agreement.
Each phase has a phase inductance cycle. Thus, two phases have such cycles which are phase shifted. By comparing the results of timing the rise in current according to embodiments of the invention, there will be substantial agreement between two points in both inductance cycles but not elsewhere. From this substantial agreement from the outcomes of the measurements, the rotor position can be derived.
One form of the invention provides a sensorless control method which can work with any power converter circuit at zero rotor speed, does not require large amounts of stored data or expensive current feedback and yet can be robust in the presence of noise on the waveforms from which it deduces position. Embodiments of the invention do not require any stored flux-linkage data, which is generally costly on memory space.
To be able to equate the times for the phase currents to reach the threshold directly, the same voltage needs to be applied to both phases. If the voltage applied is not the same as that used to provide the basic data by which to equate current rise time to rotor angle, the monitored times can be scaled by a ratio of the voltages.